The determination of moisture in materials such as liquids and solids by the Karl Fischer reaction is well known and widely used since it was first described by Karl Fischer in Angewandte Chemie 48, pages 394-396 (1935). Numerous publications have also described this technique for water determination, and reference is made to a general text by J. Mitchell, Jr. and D. M. Smith, entitled "Aquametry", published by John Wiley & Sons, 1980.
In the Karl Fischer reaction, a sample containing water is introduced into a vessel solution where it is readily absorbed. The vessel solution may be an anhydrous reagent containing sulfur dioxide, a buffer, and an anhydrous solvent, such as methanol, formamide, methyl cellosolve (a trademark of Union Carbide Co.), or mixtures thereof. A "titrant" containing iodine is introduced into the vessel solution containing the sample to be analyzed and produces a reaction of the type H.sub.2 O+SO.sub.2 +I.sub.2 =2HI+SO.sub.3.
In this Karl Fischer reaction, the water to be determined reacts with iodine on a quantitative basis; consequently, the amount of reacted iodine is a measure of the amount of water present in the sample. The titrant is generally a methanolic solution of iodine and is introduced into the vessel containing the sample and the vessel solution until no more iodine is used up. That is, an end point of the reaction is reached. Since the iodine is introduced via the titrant, the amount of titrant used is a measure of the amount of water in the sample to be tested.
Many different types of Karl Fischer reagents are commercially available and in use. For example, my U.S. Pat. No. 4,354,853 describes several of these Karl Fischer reagents, and particularly describes an advantageous "two-part" system, which is the subject of that patent application. These two parts are the titrant and the vessel solution. The titrant is essentially an iodine solution, while the vessel solution contains an alcoholic solvent, a reducing ion such as sulfur dioxide, and a buffer. A wide variety of buffers are described in that patent and in U.S. Pat. No. 4,378,972. Many of these buffers are amines. In U.S. Pat. No. 4,378,972, numerous "one-part" systems (containing all components-iodine, sulfur dioxide, and buffer-in one part) are described, as are several "two-part" systems (separate vessel solution and titrant.)
The present invention is related to improved two-part systems for the Karl Fischer determination of water in a sample. Many two-part systems appropriate for use in the present invention are described in my aformentioned U.S. Pat. No. 4,354,853, and in U.S. Pat. No. 4,378,972.
In a typical two-part system, also called a bicomponent system, part 1 of the system is the titrant. This is a solution of iodine in a solvent, for example the types of solvents described in U.S. Pat. No. 4,354,853. Part 11 of this bicomponent system is termed the vessel solution, and is usually a solution of sulfur dioxide and amine in an alcoholic solvent. The titration is carried out in a conventional titration apparatus. The titrant (iodine solution) is added to the vessel solution to modify it (pre-titration) until a certain slight excess of iodine is obtained, as indicated by suitable electrodes of the apparatus. The sample to be analyzed for water content is then added to the vessel solution and the iodine solution (titrant) is again titrated into the vessel solution until a slight excess of iodine is achieved. Accordingly, a substantial excess of iodine is not present until the titration end-point is reached. The volume of iodine solution added is a measure of the water content.